The present invention relates generally to the field of epoxy hardeners and in particular to new and useful hardeners for epoxy compositions.
Epoxy resins are used extensively in industry for the production of highly diverse articles of manufacture from aerospace structures to sporting goods. Cured epoxies have excellent adhesive strength with a variety of substrates, good to excellent strength and toughness and good resistance to solvents and chemicals. The best properties are obtained from combinations of epoxy resins and various active hydrogen hardeners which produce thermosetting copolymer systems. Epoxy resins are industrial commodities.
Epoxy hardeners are extremely diverse, including primary and secondary polyamines, tertiary amines, polyphenols, polycarboxylic acids, cyclic anhydrides, acidic polyols and combinations of these.
Mixtures of epoxy resins and hardeners and various other modifiers, diluents, fillers, etc. are cured either at ambient temperature or at elevated temperature for a time sufficient to convert the initial liquid mixture to a solid copolymer having useful properties. This procedure is referred to as the cure process or “curing.”
The cure temperature and cure time vary extensively depending on the chemical characteristics of the epoxy-hardener system. The liquid prepolymer mixture will gradually increase in viscosity over a period of time at ambient temperature. Depending on the nature of the manufacturing process, the time for the viscosity of the epoxy-hardener mixture to increase to a predetermined value is referred to as the “latency period”, “pot life”,or “joint open time.” As a practical matter, these times determine the time available for various adhesive bonding processes involving metal or composite adherends or fiber-reinforced composite materials or the cycle times of various thermosetting molding processes.
Increasing emphasis on reducing manufacturing costs has focused attention on the development of epoxy-hardener systems having relatively long latency periods combined with relatively short cure times at low temperatures. This combination presents some very difficult design problems. In general, cure times and latency periods track together; increasing the latency period by changing a formulation generally results in an increase in the cure time. It is only by changing the chemical characteristics of the hardener system that increased latency combined with shorter cure times can be obtained.
Polyamine hardeners give relatively short latency periods and will undergo partial curing at ambient temperature. However, properties are improved by a postcure at a temperature above ambient. Various combinations of polyamines and tertiary, amines will cure at moderate temperatures with somewhat longer latency periods and aromatic polyamines and cyclic anhydride curing agents will provide still longer latency periods but good properties require a postcure at relatively high temperatures. New epoxy hardeners which can meet current industry requirements are needed to provide short cures at relatively low temperatures combined with relatively long periods of latency.
Latent epoxy hardeners consisting of combinations of tertiary amines and acidic polyols have been described in U.S. Pat. Nos. 6,491,845 B1 and 6,743,375 B2. The described tertiary amines were those which were commercially available at the time. However, demands for increases in latency and decreases in cure time have resulted in a search for new amine-type epoxy hardeners which can meet these new performance criteria.